cell signaling

Cell signaling (cell signalling in British English) is the biological process by which a cell interacts with itself, with other cells, and with the environment. Cell signaling is a fundamental property of all forms of life. Typically, the signaling process involves three components: the first messenger (the ligand), the receptor, and the signal itself.

In biology, signals are mostly chemical in nature, but can also be physical cues such as pressure, voltage, temperature, or light. Chemical signals are molecules with the ability to bind and activate a specific receptor. These molecules, also referred to as ligands, are chemically diverse, including ions (such as Na, K, and Ca), lipids (e.g. steroid, prostaglandin), peptides (e.g. insulin, ACTH), carbohydrates, glycosylated proteins (proteoglycans), nucleic acids, etc. Peptide and lipid ligands are particularly important, as most hormones belong to these classes of chemicals. Peptides are usually polar, hydrophilic molecules. As such they are unable to diffuse freely across the bi-lipid layer of the plasma membrane, so their action is mediated by a cell membrane bound receptor. On the other hand, liposoluble chemicals such as steroid hormones, can diffuse passively across the plasma membrane and interact with intracellular receptors.